Abstract

Glulam is almost exclusively the chosen material of timber frame structures. Of
those, three-hinged (three-pin) portal frames are incomparably the most common
type. Being both statically determinate and stable against horizontal forces
in its own plane o er both practical (basic constructive details) and economical
bene ts.
The design of the haunch allows for various solutions: it can be curved with
continuous laminates, nger jointed, jointed with steel dowels and slotted-in
plates, or built-up with a strut.
The form of the frame derives from the main load's force line. The most appropriate
forms for large spans are curved or built-up haunches, as they ful l
both functional and aesthetic aspects. Three-pin portal frames are suitable for
spans up to 30-40 meters, being the limiting factor the transport feasability of
the frame's halves.
In recent years a handful of three-hinged structures with built-up haunches
have collapsed, leaving behind a need to analyse and study the stresses and
resistance of this structures. For obvious reasons it is specially interesting to
research and clarify the fracture risk on the built-up haunches as well as to map
the stresses created by the inner frame leg through compression to the lower
edge of the frame rafter.
Today there is only one general method used to design built-up haunches,
present in both the Glulam Handbook and the German Institute for Standardization,
DIN, which is based on established practice. Both standards share similar
simpli cations and assumtions, giving surprisingly little importance to the
shear stress that occurs in the contact area of the built-up haunch.
The aim of this master's thesis was to put together a tool, in form of a
diagram, to help design the built-up haunch, with particular emphasis on the
contact area between the inner frame leg and the frame rafter, and the subsequent
shear stresses this contact creates on the rafter. To achieve this result a
sample frame rafter was calculated using 2D frame software and generic loads
and materials. The section forces obtained were then used to create a FEM
model of the built-up haunch.
This FEM model provided a clearer understanding of the behaviour of the frame
leg-frame rafter's contact area as well as yielding a map of the shear stresses
present in the joint. Finally with the use of Linear Elastic Fracture Mechanics
and the mean stress criterion a diagram-tool was created.
This report yielded two results that may be deemed of special interest:
- Both the Glulam Handbook and the german DIN standard relegate the
shear stress in the notch as merely a design checkout, never a design factor.
- Both the Glulam Handbook and the DIN completely disregard the size
e ects in the capacity of birdmouth joints. The fracture mechanics calculations presented in this report provide prove of strong size e ects, so
that current design standards signi cantly overestimate the capacity of
great-sized built-up haunches.
This conclusion could be important in practical terms, and should be
further investigated. Experimental tests should also be inlcuded in future
research.
Keywords: glulam, built-up haunches, framed joint, three-pin portal frames,
shear stress, failure modes, LEFM, ABAQUS, size e ects.